If a spot welding process using a large electric current is utilized to bond lapped areas of two metal members having different melting points, e.g., an Al-based member (aluminum having a melting point of 660.degree. C.) and an Fe-based member (iron having a melting point of 1,540.degree. C.), a nugget is formed on the Al-based member following melting of the latter due to a difference in melting points between both the members, but a phenomenon occurs that the Fe-based member is hardly molten.
If the strength of weld zone of such different members is examined, it can be seen that the weld zone shows a strength substantially equal to that of the weld zone of Al-based members, namely the same type of members in a tensile shearing test, but shows a strength, for example, of only about one sixth of that of the same type of the members in a U-tensile test.
Therefore, it is a conventional practice to employ a process in which a clad material comprised of an Al alloy layer and a steel layer is interposed between the lapped areas of the Al-based member and the Fe-based member, with the Al alloy layer located on the side of Al-based member and the steel layer located on the side of Fe-based member (see Japanese Patent Application Laid-open No.111778/1993).
However, the prior art process suffers the following problems: In a case where the lapped areas have a complicated shape such as an arcuate shape, the accommodatability is poor, and a gap is produced between the Al-based member and the Fe-based member in the lapped areas depending upon the thickness of the clad material and as a result, the places where this process can be applied are largely limited in respect of the design. In a case where the clad materials are dotted between the lapped areas, the air-tightness of the weld zone is injured by the gap. On the other hand, in a case where the clad material is mounted over the entire length of the lapped areas, an increase in weight is caused. In addition, the clad material is relatively expensive and hence, an increase in manufacture cost of the bonded product cannot be avoided.
A further attempt has been made to provide a solid-phase bonding between Fe-based and Al-based members by decreasing a welding current.
For example, Japanese Patent Application Laid-Open No. 7-214338 discloses a technique for bonding an Fe-based metal material and an Al-based metal material by a resistance welding with use of a pin made of an Fe-based metal material having a substantially T-shaped section. However, in the case of this prior art process, the pin which is pressed by an electrode to penetrate through at least one of the materials has a complicated shape. For this reason, there are problems that the manufacture cost for the pin is increased, and in the bonding operation, labors are required by positioning and handling of the pin, resulting in a poor efficiency.
Further, the surface of the Al-based member is covered with a firm oxide film and for this reason, an enhancement in a bond strength to be provided by the solid phase bonding is hindered by the oxide film.
To avoid this, it is necessary to subject the Al-based member to an oxide film removing treatment, e.g., a brushing using a wire brush. However, the carrying-out of such a treatment is undesirable, because it increases the operating steps and the operating cost.
Furthermore, Japanese Patent Publication No. 52-2378 teaches a technique for bonding materials by a spot welding, which comprises preparing a hard material having a relatively large hardness and a high melting point and a soft material having a relatively small hardness and a low melting point, forming at least one of the materials into a rounded bar-like shape, and lapping the materials onto each other to bond them to each other, while pressing them from above and below by the pair of electrodes. With this process, an oxide film generated in the surface of the soft material formed by an Al alloy, for example, can be destroyed by a plastic deformation, and therefore, there is an advantage of enhancing the welding strength. In addition, a recessed groove positioning the hard material in a predetermined position is provided in one of the electrodes pressing the hard material, and therefore, there is an advantage that any deviation in the relative positional relationship between both the materials can effectively be prevented. However, the hard material opposed to the electrode in which the recessed groove is formed is limited to ones having such a shape that can be engaged into the recessed groove, and therefore, there is a demerit that the utilization is limited.